Despite the convenience of heating offered by the microwave oven, the commercial success of many microwavable food products has been limited by their unevenness of heating, and by the inability of their packaging to control power absorption, provide selective heating, or yield consistent browning and crisping results. For food loads shaped as slabs, non-uniform heating is widely observed as hot peripheries and cold central regions, and as patterns of lobe-like hot spots. In frozen foods, the unevenness of product temperature distributions is exacerbated by an enthalpy requirement of thawing that can exceed the energy needed to bring the food once thawed to a typical target temperature of about 70.degree. C. When an uneven deposition of microwave energy is applied to the combined enthalpy requirements of heating a frozen food, larger temperature variations are observed than in the heating of refrigerated products. Temperatures measured at the edges of the food will often exceed 100.degree. C. before its central regions have thawed. On the extended heating of frozen and refrigerated foods, temperatures tend to cluster near 100.degree. C. because of a large evaporative energy requirement in the range of 2,260 J per gram of weight loss. While this clustering of temperatures may give the semblance of improved heating uniformity, uneven energy deposition instead appears as weight loss variations over the food cross-section. Total weight losses, expressed as a proportion of the initial weight of a product, will often obscure high localized moisture losses rendering the edges of the product tough or unpalatable.
Non-uniform heating of a variety of loads ranging from frozen and refrigerated foods to ceramics can be better understood by considering the loads when in microwave-transparent containers as dielectric resonators, and those in metal-walled containers as filled waveguide or cavity resonator systems. Multiple reflections at the interfaces of a load and the air of a surrounding cavity, or at the metal walls of a container, combine to give constructive or destructive interference between opposing faces of the load. Constructive interference can be referred to as resonance (or in an adjectival sense, as resonant), and destructive interference as anti-resonance (or adjectivally, anti-resonant). For convenience, the term "resonator" herein refers to structures supporting resonant or anti-resonant effects. In simple resonator geometries, the field distributions resulting from multiple reflections can be resolved as modes, or eigenvector solutions of Maxwell's equations with characteristic eigenvalues.
There is extensive literature describing the properties and applications of dielectric resonators, as exemplified by the edition of D. Kajfez and P. Guillon, Dielectric Resonators, Artech House, 1986. Dielectric resonators are typically formed from ceramics, such as TiO.sub.2 and titanates. Air-filled metallic waveguide and cavity structures are widely used in the art, and their properties are discussed in such texts as N. Marcuvitz, Waveguide Handbook, first published by McGraw-Hill in 1951 and reprinted by Peter Peregrinus, 1986. In general, waveguide and cavity walls are chosen to be highly conductive, and the art-recognized assumption of walls that are perfect electric conductors allows the enclosed field distributions to be described by means of individual or superposed waveguide modes. The transverse field distributions of metal-walled containers resemble those of the corresponding metallic waveguide or cavity cross-sections. However, in contrast with air-filled waveguide, load dielectric constants greater than unity permit the propagation in metal-walled containers of high order modes that would ordinarily be rapidly attenuated. For loads in microwave-transparent containers, the assumption of perfectly magnetically conducting walls allows field distributions in their bulk regions to be approximated using a similar set of waveguide modes.
The resonances of food loads in microwave-transparent and metal-walled containers are discussed in a paper by R. M. Keefer The Modelling of Foods as Resonators, In Predicting Microwave Heating Performance, given at the 22.sup.nd Annual Symposium of the International Microwave Power Institute, 1987, and also in the article, R. M. Keefer, The Role of Active Containers in Improving Heating Performance in Microwave Ovens, Microwave World 7(6), 1986. The presence of higher order modes and their superposition allows load field distributions and energy deposition to respond flexibly to the boundary conditions imposed by the container and its surroundings. Unfortunately, this responsiveness also leads to an undesirable sensitivity of load heating distributions and power absorption to design of the surrounding cavity and positioning of the load within it. When combined with the large number of consumer microwave ovens, this sensitivity causes many microwavable foods to perform unreliably in delivering the desired sensory attributes, or in exceeding the minimum temperatures needed for microbiological safety.
While waveguide modes offer a useful approximate description of load field distributions and energy deposition transversely to the walls of microwave-transparent or metal-walled containers, it is important to note that the assumption of perfectly electrically conducting or perfectly magnetically conducting walls confines their dependence on load dielectric properties to the perpendicular part of the corresponding waveguide solutions. In other words, the transverse part of the waveguide solutions varies harmonically with the load cross-section, but not with the load dielectric constant. In the dependence of the structures of the present invention on load dielectric properties and the presence or absence of a load, this leads to important distinctions over the prior art. Many practical loads are shaped as slabs, that is, with at least one set of opposing faces in a substantially plane-parallel relationship. When describing propagation through or between a single such set of opposing faces, "vertical" herein refers to the direction perpendicular to the faces, although it will be understood that the present invention is not limited to any particular orientation of loads within an enclosing microwave cavity. The dependence of the vertical part of waveguide solutions on load dielectric properties has been described in the art in reference to vertical variations of power absorption. Variations of power absorption in the vertical axis of metallic containers were observed in a paper by R. M. Keefer, Aluminum Containers for Microwave Oven Use, in the Proceedings of the 19.sup.th Annual Meeting of the International Microwave Power Institute, 1984, pp. 8-12. They were also described in U.S. Pat. No. 4,990,735 to C. Lorenson et al (issued Feb. 5, 1991), incorporated by reference herein. According to Lorenson et al, load power absorption shows strong vertical variations, with maxima and minima repeating on an interval determined from the real and complex parts of the load relative dielectric constant. For convenience of description, the term "vertical resonances" herein refers to vertical variations of power absorption through one or more layers of a load. The transverse field distributions described in this patent are primarily attributed to harmonic considerations such as the order of the modes in a transverse sense, or the presence of reflective mode-clamping structures. In the context of lossy dielectric slabs, vertical variations were referred to in an article by W. Fu and A. Metaxas, A Mathematical Derivation of Power Penetration Depth for Thin Lossy Materials, Journal of Microwave Power, 27(4), 1992, pp. 217-222, incorporated by reference herein. This article also shows the concept of penetration depths used in describing load power absorption to be applicable only to loads "so thick that one can neglect the effects caused by waves reflected from the material boundaries."
The principles of geometrical optics are also instructive in understanding the present invention. The applicability of these principles to microwave problems can be seen from such texts as G. L. Lewis, Geometric Theory of Diffraction for Electromagnetic Waves, Peter Peregrinus, 1976. Snell's law of refraction provides that for loads with high dielectric constants, energy penetrating the surfaces of the loads will be directed nearly perpendicularly thereto for a wide range of angles of incidence (i.e. modes). Consistent with this observation, multiple transverse mode structures can produce similar vertical variations in high dielectric constant loads such as foods in the thawed state. Even when the individual modes cannot be readily distinguished in transverse heating distributions, simple vertical patterns of fluctuating of power absorption are often observed. Taken together with the responsiveness to applied conditions allowed by the superposition of modes, this suggests that the vertical part of the waveguide solutions provides the main restriction in determining such heating effects as overall power absorption.
The importance of dielectric properties in determining heating performance follows from the foregoing discussion of load resonances. In lossless metal-walled cavities, the resonant frequency of each mode is proportional to the inverse square root of the dielectric constant, although this is only approximately true for dielectric resonators. At a fixed frequency, changes of dielectric constant shift the dominant modes into or out of resonance, or promote the propagation of other modes. Frequency-stability is a design goal of resonators used in filter circuits, and dielectric materials are selected for minimal temperature-dependence in such applications. By contrast, large variations of dielectric properties are typically encountered in microwave heating applications. These can result from changes of load state or composition over the heating cycle, and for loads subject to dielectric relaxation phenomena, can be attributed to temperature-dependence both of their static dielectric constants and critical frequencies. The variation of dielectric properties with temperatures appears in a variety of articles and texts, for example, H. Frohlich, Theory of Dielectrics: Dielectrics and Loss, Oxford University Press, 2nd edition, 1958. From U.S. Pat. No. 4,990,735 to Lorenson et al, power absorption of a load fluctuates vertically with maxima and minima repeating on an interval determined by the real and complex parts of the load relative dielectric constant. Taking the dielectric properties of water as representative of many high water activity foods, the real part of the relative dielectric constant of water at a frequency of 2.45 Ghz varies approximately from 4.2 in the frozen state, to 82.19 in the liquid state at 0.degree. C. and 55.32 at 100.degree. C. The imaginary part of the relative dielectric constant of liquid water shows a nearly tenfold decrease from approximately 23.64 at 0.degree. C. to 2.23 at 100.degree. C. Applying such variations of load dielectric properties to the vertical intervals described by Lorenson et al, it is apparent these intervals and the corresponding power absorption will shift significantly with the temperature changes occurring over the heating cycle.
In a broad sense, the dependence of load resonances on dielectric properties leads to variability of the corresponding heating distributions and power absorption when the dielectric properties of the load are temperature-dependent. This has important consequences on the reliability of prior art structures in modifying the microwave heating of foodstuffs and other loads. As used adjectivally herein to describe microwave packaging, container, or utensil structures, "active" refers to structures incorporating microwave-reflective components intended for modifying energy deposition within an adjacent foodstuff or other load. These devices typically use such active components as patterned foil, or metallic plates or rods to provide shielding, selective heating, or localized searing effects. Additionally, susceptors and coatings containing conductive or lossy particulates are used to provide browning and crisping effects. Even for simple shielding devices found in the earlier art, an intended reduction of power absorption may be offset by the resonant enhancement of the heating caused by inadvertent selection of a resonant load thickness. Similarly, devices intended to provide increased power absorption by means of impedance-matching or coupling may fail to perform as claimed because of vertical interference effects causing a reduction of field intensities within the load. For active devices directed at a particular load or load condition, changes of dielectric properties attendant on heating may render them ineffective. These problems may be obscured by the practice of evaluating package heating performance using aqueous gel food simulants near room temperature, often without consideration of the temperature-dependence of their dielectric properties, or that such simulants are not representative of food in the frozen state. Given the large changes of dielectric constants accompanying thawing, active devices for use with frozen foods may be ineffective in modifying the heating of refrigerated foods, or the foods once thawed. Because of coupling or decoupling with load resonances, or changes in load dielectric properties over the heating cycle, devices using microwave-reflective strip components, or with reflective sheets incorporating slot or aperture perforations, may shift in or out of resonance with adverse or unforeseen consequences. In particular, on shifting into resonance, open metallic strips may arc or cause scorching of supporting materials such as paperboard. On shifting out of resonance, components dependent on the induction of strong fringing fields for browning and crisping of adjacent foods may cease to function as intended.
In response to these problems, the present invention recognizes the changes of load vertical resonances and dielectric properties occurring over the heating cycle. While extending to embodiments capable of modifying load heating performance over the entire heating cycle, it principally includes active structures that are responsive to the features of load design affecting the resonances thereof, to changes of load dielectric properties with temperature or accompanying changes of state, composition, or density over the heating cycle, to the presence or absence of loads, and to the presence or absence of adjacent dielectric materials, such as packaging, utensils or containment apparatus, or dielectric components of an external microwave cavity or oven. While changes of load resonant or dielectric properties have caused unreliable operation of prior art devices, the responsiveness of the structures of the present invention to the load and its surroundings instead provides novel features of control in modifying load heating performance.